Apparatus and processes currently used for curing thick pigmented coatings on furniture pieces are typically designed to cure the coating at a linear rate of about 8 meters/minute. In a first step 2-3 mils (100-125 microns) of a UV curable composition may be applied by spraying, roll coating or curtain coating for example, on a substrate such as a medium density fiber board (MDF). After coating, the MDF is introduced into a flash-off tunnel to remove residual volatile solvents the concentration of which typically ranges from about 30% to about 40% by weight. The residence time is usually 3 minutes at approximately 50.degree. C. in circulating air. To maintain a 3 minute dwell in the flash-off tunnel at 8 meters/minute a tunnel 24 meters long is required.
After the residual solvent has been removed from the coating, the substrate is introduced into a tunnel for initial curing. This tunnel may contain, for example, TL03 lamps having these trate designations and peaked at 420 nm alternating with TL05 lamps operating in 360 nm range of the UV spectrum. Such lamps, manufactured by N. V. Phillips, are low pressure lamps each of which produces 120 watts of energy and is 48 inches long. This initial curing step also requires a 3 minute dwell time, and to maintain the rated line speed of 8 meter/minute, a 24 meter long tunnel is required. These low pressure lamps may be positioned, for example, on 12.7 cm centers along the entire length of the 24 meter tunnel. This length requires 192 low pressure lamps to carry out the initial curing phase of the process.
At this point the coating is soft and undercured at the surface. To impart the hard, scratch-resistant properties required of this coating, a final cure phase is required. The final cure is accomplished using medium pressure UV lamps operating at 80 w/cm. Three rows of lamps can achieve a line speed of approximately 2.5 meters per minute. For example, the first row of lamps may have an output peaked at 420 nm while the next two rows of lamps may have a 365 nm peak. In order to reach a desired production rate of 8 meters/minute, a total of 9 rows of 80 w/cm lamps is required, the first 3 rows having lamps producing the 420 nm energy, and the following 6 rows producing the 365 nm output. The total lamp input power per unit area is about 65 joules per square centimeter of cured coating. About 1/5 of this energy comes from the TL lamp. The light shielding and conveyor system to accommodate 9 rows of lamps is approximately 4 meters long.
The combined length of the flash-off tunnel, the initial cure zone and the final cure station is approximately 52 meters.
In some cases particularly when the pigment coating is white, in order to obtain the desired level of opacity, multiple thin layers must be applied using the above process.
When the low intensity TL lamps are employed in the initial cure zone of the process, the coating which is applied to the edge of the substrate may be tacky to the touch or in some cases may even be wet and uncured. In many cases, to obtain a complete cure on the vertical edges of the MDF board, a secondary cure mechanism is employed.
Typically cobalt peroxide is added to the coating composition and cure on the vertical edges is then completed in a matter of hours; however, the addition of cobalt peroxide to the coating has some decided disadvantages: (a) the pot life of the composition is greatly reduced, making it imperative that the coating equipment be thoroughly purged of coating material by the end of the work shift; and (b) the cobalt peroxide may cause premature gelation of the coating in the application equipment making laborious and time-consuming cleaning necessary, and causing the loss of valuable production time.
Additional disadvantages of the above described equipment and process are: (a) the high floor space requirement, which precludes its installation in small job shops, (b) high maintenance costs due to the large number of lamps which are needed, (c) the large number of parts in process at any given time, and d) in going through the curing line the article is heated sufficiently so that it must be cooled when it comes off the line so that substrates do not stick together when stacked.